Two phenomena appear to be concerned with mechanisms controlling the rate of synthesis of progesterone in the placenta. These phenomena; end product inhibition of delta 5 minus 3 beta hydroxysteroid dehydrogenase, and an apparent non-steroid inhibition of the same enzyme, have been demonstrated in man, rat, and rhesus monkey. It is proposed to expand the studies to other species in order to determine the extent of these phenomena. Arrangements have been made for the collection of placentas from chimpanzee, sheep, horse, cow, cat and dog. After elucidation of optimum conditions for progesterone synthesis in tissue homogenates from these species, preparations will be examined for the presence and nature of steroid and non-steroid inhibition. Detailed kinetic studies on human placental microsomes have shown agreement between experimental results and theoretical calculations based on the assumption of an endogenous non-steroid inhibitor. Further work will be directed toward physical separation and characterization of the substance, using standard enzyme purification techniques.